The present invention relates generally to an electrical safety device and more specifically to a temperature control device utilizing a sensing conductor to provide overtemperature and mechanical damage protection for extension cords, electrical power cords, and electrical components. The sensing conductor is housed within electrical power cords, cables and components so as to sense temperature changes or mechanical damage over a length of the electrical cable or over the surface of the electrical component case or internal parts.
Electric powered devices are subject to numerous conditions which can result in fire, injury by burning, injury by shock or release of toxic fumes. For example, aging or mechanically damaged electrical insulation results in local shorts and overheating due to large currents. Another frequent cause of overheating in electric motors is mechanical overload or stalling resulting in excessive current. Electrical or electronic devices which are fan or conduction cooled will overheat if the cooling medium or transport method fails. A safe, reliable and inexpensive safety device is needed to sense dangerous electrical conditions such as overheating and mechanical damage which threaten life and property.
Numerous devices for sensing overtemperature conditions have been previously disclosed and claimed. Up to now, these safety devices have not enjoyed widespread use due to cost, complexity, electrical or mechanical limitations as well as safety concerns for the device itself.
Some previously disclosed safety devices depend on overcurrent protection to sense overheating or fire. This type of protection may not be effective for all parts of a device, especially if the problem is mechanical damage, such as the breaking of some of the strands of a current carrying wire or a defective electrical connection due to corrosion or poor contact. A section of the device or cord may become dangerously overheated without tripping the overcurrent protection device.
Other safety devices sense the temperature at points along the electric cord or at points within the electrical component. For example, fusible links which melt and open a circuit upon overtemperature conditions have been used. Other devices employ thermistors, RTDs or other temperature sensitive elements which, in conjunction with a sensing and control circuit, monitor temperature of the sensor and reduce or cut power to the device if the sensor overheats. Because these devices detect overheating conditions only at certain points, the safety devices protect only discrete locations; dangerous overheating conditions at other unprotected points may go undetected. In addition, protection of a long electrical cord or a number of components is not practical using thermistors, RTDs, or junction devices due to weight, bulk, and cost.
Linear sensing means provides protection over a continuous length, thereby sensing safety problems over the length of the electrical cord. Linear sensing means can potentially reduce weight, bulk and cost in many applications. However, linear sensing has several drawbacks. If electrical impedance change in a conductor as a function of temperature is used as the sensing method, a large temperature change in a short length of the conductor is equivalent in impedance change to a small temperature difference for the entire length. This leads to difficulties if the sensing circuitry is made sensitive enough to respond to a localized high temperature condition; a small ambient temperature change will cause unwanted alarm, or trip of the unit.
Safety devices which detect temperature over a distance have been used in applications such as electric heating blankets or in high voltage transmission lines. Some linear detection devices utilize specialized dielectric coatings between conductors which change electrical characteristics as a function of temperature, but these materials add to the cost, complexity, and, in some cases, reliability problems due to environmental, mechanical or aging effects on these materials. Other linear detection devices utilize line voltage AC applied to the sensor wire which can present a safety hazard for electric shock. These safety devices which detect overtemperature over a distance are not designed to detect mechanical damage which could lead to shock or fire dangers.
In U.S. Pat. No. 4,577,094 issued Mar. 18, 1986, a sensory and control device for electrical heating apparatus is disclosed and claimed. The sensory function is performed by a linear conductor with a positive temperature coefficient, placed so as to sense and control heat produced by a heating element. No method is disclosed for ambient temperature compensation. Furthermore, the sensory wire is not disposed to detect mechanical damage to the protected device.
In U.S. Pat. No. 2,501,499 issued Mar. 21, 1950, an electric heating control device is disclosed which incorporates ambient temperature compensation. No method is disclosed for sensing mechanical damage to a device.
Until now, no device has been available for wide application in electric power cords, extension cords, and other electrical devices to sense overtemperature or mechanical damage over a wide area of an electrical component and which is reliable, inexpensive, and safe. The needed safety device should utilize low cost sensing wire and low cost manufacturing processes. It should provide the safety of low voltage sensing. It should compensate for ambient temperature to reduce spurious trips and improve sensitivity. And it should provide expanded mechanical damage sensing protection to further protect the user from electrical shock as well as fire hazards.